NOTE · 09/26
Didn't set up, and then it clouded over anyway
Checked what the Moon was doing tonight and decided not to bother. Then it clouded over. First time a decision not to observe has been confirmed by the sky an hour later.
What was on offer. A 5.5 per cent waxing crescent, 27 degrees from the Sun, 31 arcmin across. 26.8 degrees up at sunset, 15 degrees by 19:00, set at 20:18.
Why a young crescent is always a bad target, and it is geometry, not luck. The Moon is only 27 degrees from the Sun, and 27 degrees of sky is about an hour and three quarters of rotation. That is the entire budget, and inside it the two things I need pull against each other:
| altitude | sky | |
|---|---|---|
| 17:30 | 32.7° | Sun still 6° up, no contrast |
| 18:30 | 21.0° | usable |
| 19:00 | 15.1° | dark, but into the murk |
High and dark never overlap. Every night a thin crescent is available it will be low whenever the sky is finally dark, which is the same airmass problem as [[altitude-beats-everything]] except that here it is baked into the target rather than into when I chose to shoot.
It would not have fitted anyway. The Moon is 31 arcmin; the SV205 on the Mak sees 12.1 by 8.0 arcmin. A third of the diameter. Any lunar night from now on is terminator sections or a mosaic, never the whole disc in one frame — worth knowing before carrying everything outside.
The mosquitoes settled it. After last night I was not going out for a 15-degree sliver that does not fit in the frame. Still no repellent in the house.
The 18th is the night
The Moon moves about 13 degrees further from the Sun each night, so it sets roughly 50 minutes later and sits higher once the sky is properly dark. By Friday:
| tonight | Fri 18th | |
|---|---|---|
| altitude at 18:30 | 21° | 77° |
| altitude at 20:00 | set | 60° |
| illuminated | 5.5% | 46% |
| elongation | 27° | 85° |
Four hours of usable sky instead of forty-five minutes, half the airmass, and a terminator running the full length of the disc instead of a sliver. Venus is still 28 degrees up at 19:00 that night, so both targets in one session.
Reminder set for 18:00 on the 18th, which gives the Mak its 45 minutes to cool before the good window opens at 18:45.
Buy repellent before Friday.